The present invention relates to a spark plug used for the ignition of an internal combustion engine, an insulator used in the spark plug as well as a process for fabricating the insulator.
In recent years, with the trend of higher output power of internal combustion engines used for automobiles and the like, the area occupied by suction and exhaust valves within the combustion chamber has been increasing. On account of this, the spark plug for igniting air-fuel mixture is required to be smaller in size, and besides the temperature in the combustion chamber has a tendency to increase due to turbochargers or other supercharging equipment and the like. Therefore, as insulators for spark plugs, those made of alumina base insulating materials, which are superior in thermal resistance, are widely used. Another reason that alumina base insulators for spark plugs are used is that alumina is superior in voltage endurance characteristics at high temperatures. However, in recent years, because the insulator tends to be thinner in thickness with the aforementioned miniaturization of spark plugs , and insulators more superior in voltage endurance characteristics are demanded.
For example, in recent years, insulators in which the alumina content is increased to 85 wt %, in some cases, 90 to 97 wt % for improvement in voltage endurance characteristics have been used (hereinafter, insulators having such high alumina contents will be referred to as high alumina insulators). However, in the present technical background, effects of the improvement in voltage endurance characteristics have not been achieved so remarkably for the increase in the alumina content. The reason of this could be that in conventional high alumina insulators, materials have not been sufficiently densified due to lack of sintering aid components, or even if densified, minute open voids are remaining in relatively large amounts so that effects of increasing in the alumina content on the voltage endurance characteristics are reduced.
Therefore, in Japanese Patent Laid-Open Publication SHO 63-190753, there has been disclosed an alumina insulator in which fine alumina powder having a mean particle size of approximately 0.1 to 0.5 .mu.m is used as a raw material, to which at least one of Y.sub.2 O.sub.3, MgO and La.sub.2 O.sub.3 is blended as a sintering aid, so that the alumina content is raised to approximately 95 wt % with the result that the voltage endurance characteristics can be improved correspondingly. As the reasons of the improvement in the voltage endurance characteristics, the publication describes that the insulator is less prone to initial deterioration by virtue of the formation of a high-melting-point grain boundary phase based on the aforementioned sintering aid components, and that the formation of the grain boundary phase suppresses the growth of alumina crystal grains, making the structure microfine, with the result that grain boundary portions serving as electrical conduction paths are elongated and bypassed.
However, in the insulator of this patent laid-open publication, because the mean particle size of alumina crystal grains is as microfine as 1 .mu.m or less, there is a tendency that large amounts of residual voids that adversely affect the voltage endurance are involved in the insulator. Further, the publication also describes that the voltage endurance is improved notwithstanding high rates of voids by virtue of the formation of the high-melting-point grain boundary phase. However, it is essentially impossible to completely eliminate the effect of voids, and the upper limit of the content of alumina directly contributing to the improvement in voltage endurance could be around 95 wt % as shown in Examples of the patent laid-open publication. In conclusion, with alumina-richer compositions adopted for further improvement in voltage endurance, the rate of voids would increase more and more, while the high- melting-point grain boundary phase that suppresses the effects of the increased rate of voids, so that satisfactory voltage endurance characteristics could no longer be expected.